I finally have the first modular instalment of my battery complete: 70 cells in a 7s10p configuration made of seven “packs”. The pack construction has been a bit slow, but along the way I’ve built some construction jigs and optimised the process. I think I could do the next set of packs in less than half the time!

These are only half packs, because my design is to have 20-cell packs as the basic battery building block. This means that they don’t “stand up” as intended in the final design – but I don’t have the battery rack built yet anyway, so I’m happy for them to lay flat on their sides for now.

In place of a proper battery management system (BMS) I’m using a hobby charger that can do a balance charge (monitor the voltage of each of the 7 packs individually). This is why each “join” jumper between the packs needs to have a separate return wire.

In keeping with the “start cheap and grow” philosophy of this project, I picked up a second-hand 24 V inverter for $50. It has a rated output of 600W, but is a modified sine wave inverter which is not great for  sensitive loads. As a proof of principle I used the 240 V power from the inverter to power the lights for these photos. With my current battery size I can’t even use the full 600 W as it would draw more than 2 A per cell (I’ve designed and tested for 1 A max).

It was a great feeling to finish the wiring harness and hit that switch. There’s a long way to go before the battery is really useful, but it is now ready to power things and let me move into a fun new stage of testing and design. I can return to cell extraction, and while I’m accumulating the next 70 good cells I can tackle the next big challenge: charging this battery from the sun!